Photographic, silver halide elements having very high contrast are conventionally used in the field of reprography for the production of screened images from halftone recording elements, in photo typesetting technology, in line transparencies and photomasks among others. By the term "ultrahigh" in photographic systems I mean that the contrast is much higher than can be normally expected if it is assumed that each individual emulsion grain is exposed and developed independently of its neighbors. Ultrahigh systems utilize, for example, effects in which the development of a grain will initiate the development of a neighboring grain even if this grain was not exposed sufficiently to be developable by itself. This phenomena is conventionally known by the term "infectious development" and is well-known to those skilled in the art.
There are a number of conventional photographic systems which produce the aforementioned effects. These are the litho systems wherein a great majority of the grains within the silver halide element are silver chloride grains and these systems are developed in typical hydroquinone-containing developers at a relatively high pH. Additionally, these developers are characterized by a low sulfite content and the absence of any superadditive developer compounds. The problem with these litho systems is that they are relatively slow and require an induction period before the infectious development and the high contrast are achieved.
Silver halide photographic systems which employ hydrazines have been proposed. These hydrazine-containing systems, which are well-described in Research Disclosure 23510, November, 1983, and the references contained therein, are usually based on phenylformylhydrazine and derivatives thereof. These hydrazine-containing systems can produce the desired ultrahigh contrast effects and are considerably faster than conventional litho systems. However, these known hydrazine-containing systems also require higher pH development and it has always been a desire within the industry to reduce the pH of developing solutions because of the effect on the environment. Additionally, the image quality of these films is substantially poorer than conventional photographic litho elements.
Ruger, U.S. Pat. No. 4,937,160, the pertinent disclosure of which is incorporated herein by reference, describes a certain class of aryl hydrazides that will, when added to a gelatino, silver halide emulsion, produce the desired ultrahigh contrast effect in a developer solution at a lower pH than those prior art hydrazines described in Research Disclosure 23510. Many of the aryl hydrazides disclosed in Ruger U.S. Pat. No. 4,937,610 can be added to the emulsion as an aqueous solution or a semi-aqueous solution, e.g., mixture of water and an alcohol. This is a great advantage since gelatino silver halide emulsions are much more receptive to this type of solution and other hydrazines must be added dissolved or mixed in organic solvents. The problem with the use of these aqueous solutions of the said aryl hydrazides described by Ruger is that they have a limited shelf-life, e.g., a few hours or less, and thus must be frequently renewed. Additionally, these compounds are still only partially soluble in a mixture of alcohol and water and only very dilute solutions are possible. Thus, it is desirable to increase the shelf-life and increase the concentration of these aqueous solutions and to improve the stability thereof.
It is an object of this invention to provide a method for stabilizing aqueous solutions of aryl hydrazides which are to be added to gelatino silver halide emulsions.